Is there a preferred type for an Observable with no need for a value in Next events?

Question

I have an Observable which is only used for triggering flatMap/map. So I only ever need the Next event and never a value. I could use my own concept for such a trash value, but I'm wondering if there's an RxSwift convention for it.

Here's what I'm dealing with:

// I'd rather not have an Element type that someone might use
let triggeringObservable: Observable<SomeSessionClass> 

// ...

triggeringObservable.map { _ -> String in // The actual value is ignored
    return SomeLibrary.username() // `username()` is only ready when `triggeringObservable` sends Next
}

In this example, triggeringObservable is rx_observer on some property in the library which will let us know that username() is ready to be called.

Solution 1

You can simply use an Observable<Void> for this purpose. Like so:

   let triggerObservable = Observable<Void>.just()

    triggerObservable.subscribeNext() {
        debugPrint("received notification!")
    }.addDisposableTo(disposeBag)

or in your example:

let triggeringObservable: Observable<Void> 

// ...

triggeringObservable.map { Void -> String in // The actual value is ignored
   return SomeLibrary.username() 
}

Solution 2

In Swift 4 (and probably earlier) you can pass an empty tuple as an argument to a method expecting a Void associated type.

var triggerObservable = PublishSubject<Void>()
...
triggerObservable.onNext(()) // "()" empty tuple is accepted for a Void argument

Or you can define an extension to overload onNext() with no args for the Void case:

extension ObserverType where E == Void {
    public func onNext() {
        onNext(())
    }
}

...
triggerObservable.onNext()

Solution 3

This worked for me on Swift 4.1:

Observable<Void>.just(Void())

Solution 4

This worked for me on Swift 4.2:

Observable<Void>.just(())

View more solutions

Comments

[Womble]

In Swift 5.1, I had to explicitly pass a Void() to onNext(). Cheers.

[Martin Berger]

Your solution enables multiple events with one subsrciption. The ones with Just are single use only. +1